The present invention concerns a method in a press section of a paper machine, for detaching a web from a smooth face of a press roll.
The present invention also concerns a device making use of the method in accordance with the present invention in the press section of a paper machine, the press section including a smooth-faced press roll, preferably a central roll. The web can be detached from the smooth face of this roll and then passed to a drying section of the paper machine.
So-called closed press sections are commonly used in a paper machine, in which one press nip or, as a rule, several press nips is/are formed in conjunction with a central roll. An example of such a previously-used press section, is a press section marketed by the assignee under the trademark "SYM-PRESS II", where a smooth-faced central having a larger diameter than diameters of the other press rolls, is usually make of rock, as a rule of granite. Since granite is an unhomogenous natural material of low tensile strength, it is quite questionable in machine construction. For example, if a granite roll is desirably heated, deformations thereof which are dependant upon temperature, are non-linear and difficult to predict.
Granite has relatively good properties as press roll material for detaching a web, which is at least one of the reasons for its repute. However, the detaching properties could be better, especially with regard to unbleached paper qualities.
In a manner known in the prior art, the web is detached as an open, unsupported draw from a face of the central roll in the press. This open draw is quite critical in view of the operation of the paper machine. In this open draw, a difference in speed is utilized for extending the web, which results in certain drawbacks. Moreover, the open draw forms a questionable point susceptible to breaks in the paper machine.
Prior art technology has not provided sufficient means for controlling detaching of a web from a smooth-faced central roll, and the subsequent open draw. The unfavorable properties of granite have, for their part, also made control of the detaching and the open draw more difficult. The open draw of the web has become an increasingly important and questionable point, with the continuously increasing running speeds of paper machines. Since different paper qualities are often manufactured by a paper machine, with adhesion to the face of the rock roll being different, variations in detaching tension required for a web result.
In a SYM-PRESS II press section, the properties of the surface of the central roll in the second and third press nips must be such that the moist web adheres to the roll face as best as possible. On the other hand, the web should be readily detachable from the roll face, for transfer to the drying section. Attaining these paradoxical requirements has not been completely successful in all respects, by the means known in the prior art.